#21  
Old 06-23-2011, 08:50 PM
benguilford's Avatar
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Join Date: Dec 2010
Location: Melbourne
Posts: 99
Default Re: Concrete Slab Depth and Reinforcement

I can see what you mean - I think I was picturing a small area in the centre of the slab that was affected by this, but it's the entire slab, through the full thickness.

What did the Quickcrete rep have to say? If you have followed their instructions to the letter as you said, then they clearly did not provide you with sufficient information and are thereby at fault - I'd be asking them to pay for the removal of the slab AND to provide you with replacement bagged concrete. As I've said previously, the slab will still be able to take the load, however Neil is right when he says it will eventually fail, due to water ingress and subsequent freeze/thaw cycles. Putting a layer of grout on the top won't seal the slab completely, as you can see the sides are porous as well, and presumably so is the underside. If the rep can come up with a way of properly sealing the slab (such as flooding the entire slab with low viscosity epoxy resin to fill ALL of the voids - an expensive solution) then it's worth considering, but DANG. That sucks Fermentor...
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  #22  
Old 06-24-2011, 09:41 AM
Peasant
 
Join Date: Jun 2011
Location: Ohio
Posts: 40
Default Re: Concrete Slab Depth and Reinforcement

The rep evaluated the slab and said it will be fine, as long as I coat it with the sand-mix topping. He recommended 1/4".

He said that, if I'd have smoothed the surface, I would have been fine. It would have sealed. By placing the sand-mix atop (and along the side) of the block, it will take care of the process that should have happened if I'd have troweled the top in the first place. The rep mentioned several times that, overall it "looked like a really nice job." The plastic and the gravel base should prevent water from wicking from the underside...and he felt the bottom 90% of the slab was solid anyhow.

I am going to try the topping. I've already bought all the sand-topping and bonding agent, so I don't have much to lose. I also think it will be a good idea to apply a concrete sealer annually.

I figured I'd have problems somewhere, but never with the base slab. Oh well...any project you undertake costs twice as much as you'd expect, and takes twice as long.

As always, I really appreciate everyone's comments and support. Great group of people here. I look forward to the day when I can discuss pizza and bread techniques rather than my masonic ineptitude.
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  #23  
Old 06-24-2011, 10:31 AM
Il Pizzaiolo
 
Join Date: Jan 2007
Location: Tampa, FL
Posts: 1,446
Default Re: Concrete Slab Depth and Reinforcement

That really looks like a dry mix. Its been my experience that the manufacturers grossly recommend under hydration for their premixed bag products. I'm guessing they are aiming for maximum strength, they always say a dryer mix is the strongest, but the average homeowner has no clue that he needs vibrating equipment just to pour a simple slab for a sidewalk, shed, or in this case an oven. I've done a couple of dozen small slabs with premixed bag products over the years and have had to add more water than recommended every time in order to get it flow. I won't say that I have not affected the strength, but I have never had one crack
I think the rep has you headed in the right direction. I also agree with your plan to seal any remaining exposed areas.

RT
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  #24  
Old 06-24-2011, 02:02 PM
stoveup's Avatar
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Join Date: Sep 2010
Location: Winston-Salem, North Carolina, USA
Posts: 86
Default Re: Concrete Slab Depth and Reinforcement

Your slab looks OK to me - it will very strong in spite of the voids. In fact, if it were mine, I would not put the grout layer on the top immediately - I would start the block work directly on the slab as it is. But either way, you will be fine.

I am not concerned about the rebar in your slab, either. If you had left the rebar out completely, it would still be OK. The plans for the Pompeii oven are seriously over-engineered, so there is [U]lots[U] of leeway.

I am neither a professional mason nor a masonry professional. In a previous life, I was a contractor and did a lot of concrete work, primarily for the U.S. Army Corps of Engineers. Those guys taught me more about concrete than I really want to know - including how to bust it out and do it over!

When you are ready to pour the hearth slab, don't add a whole lot more water to the concrete mix. Based on the picture, it will take only a small amount in addition to what you used for the base - add it in small amounts until you have a mixture that is easy to move around in the forms with a shovel but that does not flow of its own accord. Too much water can weaken the final product by allowing the small aggregate particles (sand) to fall out as the concrete is poured and manipulated.
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  #25  
Old 06-24-2011, 05:35 PM
benguilford's Avatar
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Location: Melbourne
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Default Re: Concrete Slab Depth and Reinforcement

That's great news Fermentor! PHEW *wipes brow*
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  #26  
Old 06-25-2011, 08:11 AM
Peasant
 
Join Date: Jun 2011
Location: Ohio
Posts: 40
Default Re: Concrete Slab Depth and Reinforcement

Quote:
Originally Posted by stoveup View Post
Your slab looks OK to me - it will very strong in spite of the voids. In fact, if it were mine, I would not put the grout layer on the top immediately - I would start the block work directly on the slab as it is.
I'm more comfortable with the block adhering directly to the slab rather than the grout layer, so I think I will take your advice. Thanks!
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